This invention relates to temperature control systems for electric surface heater.
In FIG. 1, there is shown in a schematic cross-section a conventional electric surface heater A, in which insulating layers 4 are placed in piles on respective surfaces of a layer of series connected filmy heating elements 1, a thermosensitive electrode 3a formed of such metal foil as an aluminum foil and a thermosensitive element 2 made of such sheet-shaped and electrically semiconducting plastics as a nylon resin and having a negative impedance characteristic with respect to temperature variations are placed on one of the insulating layers 4 sequentially in piles, and another thermosensitive electrode 3b is further placed on the element 2 which is also covered with another insulating layer 4. In using this heater A, a voltage is applied between the two thermosensitive electrodes 3a and 3b that holding the thermosensitive element 2 between them and any impedance variations of the thermosensitive element 2 is detected to determine and control the temperature of the heating elements 1.
In this case, a voltage of the same frequency as of the commercial current source voltage applied to the heating elements 1 is used for the voltage applied between the respective thermosensitive electrodes 3a and 3b.
The surface heater of the type shown in FIG. 1 is thus complicated in structure and high in production costs, and different type surface heaters respectively having such a simpler structure as shown in FIG. 2 or 3 have been suggested.
The heater shown in FIG. 2 is formed by placing sequentially in piles an insulating layer 4, layer of heating elements 1, thermosensitive element 2, thermosensitive electrode 3 and insulating layer 4 in the order of thus mentioned. The heater shown in FIG. 3 is substantially of the same structure as in FIG. 2, except that the thermosensitive electrodes 3' of the same shape and of the same number as of the heating elements 1 are disposed above the respective heating elements 1 to oppose each other. In these surface heaters of FIGS. 2 and 3, a signal voltage for detecting the impedance variation of the thermosensitive element 2 is applied between the thermosensitive electrode 3 or electrodes 3' and the heating elements 1.
These heaters of FIGS. 2 and 3 are of simpler structures resulting in lower manufacturing costs than in the case of the heater of FIG. 1 and are widely employed recently in combination with various type temperature control systems. Since the impedance variation detecting signal of the same frequency as the commercial source voltage is employed in these control systems similarly to the case of FIG. 1, however, there arise such problems that, due to the commercial source voltage applied to the heating elements 1, a potential of about half of the source voltage is provided to the thermosensitive electrode 3 or electrodes 3' coupled to the heating elements through the thermosensitive element 2 and the detecting signal applied between the heating elements 1 and the electrode 3 or electrodes 3' is influenced by such potential, and further that, once a short circuiting is caused to take place due to, for example, a metal made pin or the like stuck accidentally through the thermosensitive electrode and a substantially middle part of the heating elements being at the same potential as that of the thermosensitive electrode, or when a local excessive heat rise occurs at such middle part due to an external heat insulation by a cushion or the like placed on the particular part, any variation in the detecting signal for detecting the impedance variation can no longer be detected and to avoid such troubles the control system is provided with a leakage detecting function and the like, whereby the system is required to become rather expensive notwithstanding the lowered costs for the heater. The present invention has been suggested to eliminate these problems.